Process for providing a chromate coating on zinc and cadmium surface



United States Patent Delaware No Drawing. Filed Dec. 9, 1960, Ser. No. 74,755

3 Claims. (Cl. l486.1)

The present invention relates to a process and to compositions for providing dyed corrosion resistant surfaces on zinc and cadmium.

Chromate conversion coatings are normally applied to the surface of zinc and cadmium for corrosion protection by immersison in a chromic acid-containing bath. For decorative purposes, it is often desirable to dye the conversion coated surface. Chromate conversion coatings prepared by immersion in dilute chromic acid solutions are usually colorless or slightly yellowed. Such coatings are easily dyed by dipping into aqueous dye solutions. However, such coatings are not very corrosion resistant as compared to thicker chromate coatings prepared from more concentrated solutions.

It is an object of this invention to provide a process for preparing dyed corrosion resistant conversion coatings utilizing dilute chromic acid baths.

It is also an object of this invention to provide baths for dyeing relatively thin conversion coatings and at the same time imparting additional corrosion resistance to the dyed coating than obtained with the same coating before immersion in the novel bath.

The invention also contemplates providing novel compositions for the preparation of baths in which conversion coatings may be dyed and rendered more corrosion resistant.

Chrornate conversion coatings obtained from relatively dilute chromic acid baths may be dyed and at the same time rendered more corrosion resistant by immersion in an aqueous dye-containing bath which also contains chromic acid under specified conditions and concentrations. In the dye-containing bath, the dye concentration is between 0.1 g./l. and 5 g./l., and preferably between 0.4 g./l. and 1 g./l.; and the chromic acid concentration is between 0.2 g./l. and 2 0 g./l., and preferably between 1 g./1. and 4 g./l., with an optimum concentration of about 2 g./l. The pH of the dye bath must be maintained between 3 and 11 and preferably between 4.5 and 7, with an optimum pH at about 5. The pH is significant in that the effective conversion film may be destroyed by either strongly acidic or alkaline solutions. If the chromic acid concentration in the dye bath is such that the solution is too acid or alkaline, it can be adjusted upward with caustic such as potassium or sodium hydroxide, or adjusted downward with an acid such as acetic acid, nitric acid, or sulfuric acid. The dye solution may also contain relatively inactive ions such as sulfate, acetate, nitrate, borate, chloride, and fluoride which do not interfere with either the dye absorption or the improvement in corrosion protection. The ability of the dye solution to tolerate such foreign ions is important in that they are commonly present in chromating solutions and may be dragged into the dye bath. In addition, it is possible to prepare the dye-containing bath by diluting a solution of 3,147,153 Patented Sept. 1, I964 "ice a conventional (dilute or concentrated type) chromating bath and adding the desired concentration of dye.

The dye concentration is not critical although the range of 0.4 g./l. to 1 g./l. is preferable for most dyes; 0.5 g./l. is the preferred concentration for most purposes. Increasing the concentration and/ or the temperature usually increases the rate of absorption so that a specific depth of color is obtained in a shorter time. The temperature for dyeing is not critical and may range from room temperature to about C. Similarly, increasing the time of immersion usually increases the depth of color. The immersion in the dye bath may be between 5 and about seconds, and is preferably between 10 and 45 seconds. The particular dye used is dependent upon the color desired.

The dye baths are preferably made up from novel dry compositions containing from 1 to 50 and preferably 4 to 10 parts by weight of dye and 2 to 200 and preferably 10 to 40 parts by weight of CrO The source of the CrO may be chromic acid per se or such equivalent salts as the chromates, bichcromates or polychromates of sodium, potassium, ammonium, etc. Salts of relatively inactive ions such as sulfate, acetate, nitrate, borate, chloride and fluoride can be used in amounts up to the amount of chromic acid or equivalent salt. Care should be used to assure that all the salts and the dye material are dry so that reaction will not occur between the dye and chromate.

Although the solid compositions are preferred, the baths may be made up from liquid concentrates which are prepared from materials in the proportions described above for solid compounds with the dye concentration ranging from about 2 to 50 g./l. and preferably 4 to 20 g./l. and the CrO or equivalent salts ranging from about 5 to g./l. and preferably 10 to 50 g./l.

The proces is applicable to the dyeing of cbromate con version coatings prepared by dipping zinc or cadmium in dilute aqueous chromating solutions containing between about 0.5 g./l. and 5 g./l. of chromic acid, and suflicient sulfate and nitrate ions to give a CrO /SO ratio of from about 0.5 to 2, and a CrO /NO ratio of from about 0.05 to 0.2, and containing such other anions as fluoride, fluoride-complexes, borate, and/ or acetate; the pH should be less than 2 and preferably less than 1. The dip time is preferably between 10 and 30 seconds at room temperature. The temperature is not critical, usually varying from room temperature to 5 C. to 15 C. above room temperature. The source of chromic acid (CrO in the chromating bath, as well as in the dye bath may be chromic acid per se or such equivalent salts as the chromates, bichromates, and polychromates of sodium, potassium, Zinc, ammonium, etc.

For the purpose of giving those skilled in the art a better understanding of the invention, illustrative examples are given.

Zinc-plated steel panels were immersed for 15 seconds in a chromating solution prepared by adding to 1 liter of water 1.9 g. of Na Cr O -2H O (equivalent to 1.3 g./1. of CrO 1.9 g. of NaHSO (equivalent to 1.5 g./l. of SO and 15 cc. of 42 B. HNO (equivalent to 14 g./l. of N0 at 24 C. The panels were then rinsed and inserted in the dye solution specified in Table I for 30 seconds at room temperature (24 C.). The panels were then rinsed, dried, and tested for corrosion resistance, using the standard salt spray test (5% solution at 35 C.) in accordance with procedure B-117-57T of the American Society for Testing Materials. The first appearance of white zinc corrosion product is recorded in the last column of Table 1.

in a chromating solution containing between 0.5 g./l. and 5 g./l. of chromic acid and sufficient sulfate and nitrate ions to give a chromic acid/sulfate ratio of between 0.5

Table I Dye Solution Hours in Example Color salt spray No, obtained to first Dye trade name Source Chemical Amount of CrO; additives pH zinc description dye, g./l. corrosion 1 Poirrier Blue Hartman Leddou An azosalicylic 1. 25 (a).-. None 5.5 Dark 12 Co. dye. purple 1. 25 (l.\) 3 2.. NflzCl'zOrQHzO 4. 9 d 24 0.7 None 5.5 Pink 12 2 Alizarin Red S Allied Chemical Acid athraqui- & Dye Corp. none dye. 0.7 1 g./l. ClOa, 1.4 g./l. 7.0 -.do O

NHOg-CCI'IS. 0. 5 (21)-..- None 8.0 Orange- 12 gold. 0.5 .37 g./l. NEQCTZO1'2I'I20, 10.0 (10 24 3 Orange II Eastman Kodak... A monoazo dyc CrO-,-/SOt100/1.

0.5 (c) .37 g./l.Na1Cr2O -2IIzO 10.0 do 24 0. 5 ((1)-..- 5.9 g./l. KzCrzO: 7. 9 Lighlti 24 go 0.25 (it)... None 5. 5 Blue- 12 4 Iontamine Black E E. I. du Pont do A diazo dye bla k Double. Nemours. 0.25 (b) l 13Ng./ lI.ONagCrOi, 2.84 g./l. 5. 9 d0 24 at s.

1 DH adjusted with HNO:;.

2 pH adjusted with NaOI-I.

A comparison of the results noted in the table of the corrosion resistance obtained from the dye solutions containing chromic acid indicates that the improvement in corrosion resistance is roughly 100% based on the amount of time in salt spray. The dyed coatings of the present process are uniform and have an attractive appearance. The process is applicable to any zinc or cadmium surfaced article, although the largest use is in zinc surfaced steel.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. A process for providing a corrosion resistant chromate coating on zinc and cadmium surfaced metal comprising dipping said metal in a chromating solution containing between 0.5 g./l. and 5 g./l. of chromic acid and sufiicient sulfate and nitrate ions to give a chromic acid/ sulfate ratio of from about 0.5 to 2, and a chromic acid/ nitrate ratio of from about 0.05 to 0.2, withdrawing said metal and then immersing said metal in a dye solution containing between 0.4 g./l. and 1 g./l. of dye, between 0.2 g./l. and g./l. of chromic acid, having a pH between 4.5 and 7 and a temperature between room temperature and 65 C., until the desired color is obtained.

2. A process for providing a corrosion resistant chromate coating on zinc and cadmium surfaced metal comprising dipping said metal for between 5 and 90 seconds and 2 and a chromic acid/nitrate ratio of between 0.05 and 0.2, withdrawing and rinsing said metal, then immersing said metal in a dye solution containing between 0.1 g./l. and 5 g./l. of dye and between 0.2 g./l. and 20 g./l. of chromic acid, having a pH between 3 and 11 and a temperature between room temperature and C. until the desired color is obtained.

3. A process for providing a corrosion resistant chromate coating on zinc and cadmium surfaced metal com prising dipping said metal for between 10 and 45 seconds in a chromating solution containing between 0.5 g./l. and 5 g./l. of chromic acid and sufficient sulfate and nitrate ions to give a chromic acid/sulfate ratio of between 0.5 and 2 and a chromic acid/nitrate ratio of between 0.05 and 0.2, withdrawing and rinsing said metal, then immers ing said metal for between 10 and 45 seconds in a dye solution containing between 0.4 g./l. and 1 g./l. of dye, between 1 g./l. and 4 g./l. of chromic acid, having a pH between 4.5 and 7 and a temperature between room temperature and 65 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,236,549 Darsey et a1. Apr. 1, 1941 2,288,552 Siebel et a1 June 30, 1942 2,393,640 King Jan. 29, 1946 2,482,728 De Long Sept. 20, 1949 2,564,864 Thompson Aug. 21, 1951 2,902,394 Jeremias Sept. 1, 1959 

1. A PROCESS FOR PROVIDING A CORROSION RESISTANT CHROMATE COATING ON ZINC AND CADMIUM SURFACED METAL COMPRISING DIPPING SAID METAL IN CHROMATING SOLUTION CONTAINING BETWEEN 0.5 G./L. AND 5 G./L. OF CHROMIC ACID AND SUFFICIENT SULFATE AND NITRATE IONS TO GIVE A CHROMIC ACID/ SULFATE RATIO OF FROM ABOUT 0.5 TO 2, AND A CHROMIC ACID/ NITRATE RATIO OF FROM ABOUT 0.05 TO 0.2, WITHDRAWING SAID METAL AND THEN IMMERSING SAID METAL IN A DYE SOLUTION CONTAINING BETWEEN 0.4 G./L. AND 1 G./L. OF DYE, BETWEEN 0.2 G./L. AND 20 G./L. OF CHROMIOC ACID, HAVING A PH BETWEEN 4.5 AND 7 AND A TEMPERATURE BETWEEN ROOM TEMPERATURE AND 65*C., UNTIL THE DESIRED COLOR IS OBTAINED. 